What's more, it's not enabled by default and it's only temporary measure, not full and complete mitigation.

Point is that Google won't be releasing ASAP patched version of Chrome 63.Google will wait for next release cycle, so till Chrome 64, which will be released on 2018-01-23.So looks like the security is not Google priority.

AMD CPUs are safe against the current version of Meltdown, which focuses primarily on Intel’s architecture.

Depending on how the final patched for the Intel CPU kernel bug vulnerability are implemented the AMD CPU's may still suffer a performance hit as collateral damage. It was requested for Linux kernel to make an exception for AMD cpu's since they are not vulnerable to current meltdown.